Commercially competitive fuel injector systems of the future will almost certainly need some capacity for controlling the timing of commencement of injection in response to changing engine conditions in order to achieve acceptable pollution abatement and fuel efficiency. Certainly, some emission control standards will be difficult or impossible to meet unless timing of fuel injection can be variably controlled extremely accurately on a cycle-by-cycle basis depending on operator demand and/or engine conditions.
Likewise, numerous attempts have been made to design a unit injector system which provides for variable timing. For example, many conventional unit fuel injectors include a multi-plunger arrangement having a timing chamber into which a variable quantity of timing fluid is supplied to form a hydraulic link between the plungers, the length of which is varied to change the injection timing. U.S. Pat. No. 4,986,472 to Warlick et al. and U.S. Pat. No. 4,531,672 to Smith disclose representative examples of unit injectors using variable hydraulic timing links to variably control injection timing. These types of unit fuel injectors have proven themselves to be reliable and effective, and have played a significant role in improving engine emissions without sacrificing fuel economy and power. Nonetheless, the use of a hydraulic link is not without its disadvantages. The hydraulic links found in these prior art fuel injectors are subject to parasitic losses and high pressure leakage. The parasitic losses are reflected in the high temperature of the fuel (which also serves as the timing fluid) which is spilled when the timing chamber is emptied each cycle, and which otherwise leaks, back to the fuel supply system. The amount of heat absorbed by the fuel and ultimately the temperature of the fuel in the fuel supply tank has been found to increase to an unacceptably high level due to higher injection pressures. These higher injection pressures are necessary to improve fuel economy and reduce emissions thereby enabling engine manufacturers to meet strict emission standards set by recent and upcoming legislation. However, the high temperature of the spill and leakage fuel is a prime reason why many known high pressure fuel systems currently require costly, undesirable fuel coolers.
Additionally, multi-plunger type fuel injectors having a variable volume timing chamber have a variable overtravel and an inherent effect of varying the timing by changing the length of a hydraulic link is that the injection event occurs at a different point on the cam shaft when, e.g., the timing is retarded than when it is advanced. This has the effect of causing the injection event to occur at a higher camshaft velocity with an advanced engine timing than at a retarded engine timing, which results in a change in the injection pressure being associated with such a timing change. To meet future emissions and performance requirements, it now appears that it will be necessary to be able to control injection pressure independently of injection timing, thereby enabling, for example, the injection pressure to be maintained constant despite a change in timing.
A fuel injection system which does not utilize a hydraulic link for timing purposes is described in U.S. Pat. No. 4,206,734 to Perr et al. The adjustable timing mechanism for the fuel injection system of this patent utilizes a shaft having an eccentric portion which rotates to move a cam follower with respect to a cam to vary the timing of injection of a fuel injector. The mechanism is designed to ensure that the rocker arm and injector plunger remain in essentially the same position at the end of the injection stroke regardless of the setting of the timing, thus allowing the timing to be adjusted during operation of the injector while ensuring a substantially constant mechanical load on the injector. However, timing adjustment using the device disclosed in this Perr et al. patent does cause small changes in the position of the rocker arm and injector plunger, and thus, changes, to some degree, the mechanical load or "crush" on the injector body. Furthermore, this device requires the controlled rotation of the eccentric shaft to vary timing. Moreover, the actuating arrangement for controlling the rotation of the eccentric shaft, which is designed to mount to the side of the engine, requires arm and linkage arrangements imposing undesirably large space requirements.
U.S. Pat. No. 4,306,528 to Straubel et al. discloses an injection pressure and timing control device that includes a timing setting device that is connected to a multi-plunger unit injector for controlling the onset of injection, and an injector drive and timing correction apparatus connected to an injector rocker arm for correcting, or compensating for, excessive or undesired variations in the timing of injection caused by the timing setting device, thus allowing the injection pressure to be controlled independently of the engine speed. In particular, the timing setting device controls the amount of fluid in a timing chamber to form a variable length hydraulic link, and the injector drive and timing correction apparatus may include an actuator arm that pivots a rotatable eccentric shaft for advancing or retarding the injection timing by shifting a drive lever which forms a cam follower disposed between the timing cam and a connecting link acting on the rocker arm. However, this arrangement cannot assure that the rocker arm remains in essentially the same position at the end of the injection stroke regardless of the timing setting. Moreover, this device requires the controlled movement of an eccentric shaft to vary timing of injection.
U.S. patent application Ser. No. 527,346 filed Sep. 12, 1995 entitled Variable Injection Timing and Injection Pressure Control Arrangement and assigned to the assignee of the present invention, discloses a variable timing cam follower lever arrangement including a cam follower which is moved relative to an actuating cam so as to control the timing of fuel injection. However, this arrangement requires the rotation or movement of an eccentric shaft to move the follower lever.
U.S. Pat. No. 5,107,803 to Furnivall discloses a two-piece rocker arm including a cam engaging arm and a valve engaging arm pivotally mounted on a rocker shaft wherein the valve engaging arm includes oil passages and a cavity for receiving a piston. This arrangement automatically permits changes in valve timing by controlling the oil pressure in the passages and the piston cavity so as to control the relative movement between the engaging arms of the rocker arm. However, this device is used to vary valve lift and, therefore, could not be used to operate a unit injector without causing structural failure to the injector due to excessive crush loads by the injector plunger on the injector body. U.S. Pat. No. 5,113,813 to Rosa discloses a similar device for providing variable valve timing and lift.
Consequently, there is a need for a cam follower lever assembly capable of effectively operating a unit fuel injector or an engine valve without significantly affecting injector plunger travel and valve lift respectively, while also permitting variable control of the timing of injector or valve operation.